The whole idea of overlay crochet is to create a base, a background with simple stitches (single crochet stitches are generally used), and add long, complex stitches above. The background and overlay image are crocheted at the same time!
So the most important thing about overlay crochet is that the stitches of the background are always crocheted in back loops only (unless otherwise stated in the pattern) while the overlay stitches are added (“anchored”) in the front loops of stitches of underlying rounds/rows.
Beginner, Intermediate or Advanced?
Attaching new yarn
Crochet in back or front loops
"Anchoring" overlay stitches
Let’s see it in the pictures.
First picture shows a circle of sc with all the rounds crocheted in back loops only (BLO). You can notice that front loops (FL) stay unused. (You can also see that every stitch has its own front and back loop). And second picture shows the same circle but worked through both loops. You see? They are no “free” front loops!
If you crochet the background through both or front loops you won’t be able to place overlay stitches.